1. Field of the Invention
The present invention relates to a sensor that introduces ambient air into the inside of the sensor via a ventilation filter.
2. Description of the Related Art
Conventionally known is a sensor including a long detecting element that detects a gas to be measured, such as O2 or N2, a main fitting that holds the detecting element, and an outer tube that is arranged at the rear end of the main fitting and accommodates a rear end portion of the detecting element therein.
Such a sensor includes a lead wire that is arranged inside the outer tube from outside the outer tube through the rear end opening of the outer tube and is connected to the detecting element, and an elastic member (hereinafter also referred to as a grommet) that is arranged inside the rear end opening of the outer tube so as to hold and cover the radial periphery of the lead wire.
Among sensors configured in this manner, for example, an ambient air introduction type sensor, such as an oxygen sensor, introduces ambient air into the inside of the sensor. The physical quantity of the introduced ambient air serves as the basis for detecting the physical quantity of an object to be measured. Further, a through hole for introducing ambient air into the inside of the sensor is provided in the grommet. Also, a ventilation filter having permeability and a waterproofing property is arranged so as to cover the rear end opening of the through hole. This configuration secures ventilation without allowing moisture to pass into the inside of the sensor through the ventilation filter (for example, refer to Patent Documents 1 and 2).    Patent Document 1 JP-A-2000-249678 (FIG. 2)    Patent Document 2 JP-A-2002-286682 (FIG. 10)
3. Problems to be Solved by the Invention
However, the ventilation filter described in Patent Document 1 is sandwiched between an inner peripheral surface of the through hole provided in the grommet and a tubular insertion member inserted into the through hole, and the end of the ventilation filter is arranged inside the sensor. For this reason, there is a possibility that moisture adhering to the surface of the ventilation filter may enter inside the sensor through the gap between the inner peripheral surface of the through hole and the ventilation filter.
Additionally, the ventilation filter described in Patent Document 2 covers almost the entire upper surface of the grommet, and a protective member is provided so as to cover the ventilation filter. The protective member is fixed between the outer tube and the grommet, and the end of the ventilation filter is arranged in the space between the protective member and the grommet, the space is blocked from the outside. For this reason, when moisture adhering to the ventilation filter seeps into the space between the protective member and the grommet along the surface of the ventilation filter, there is a possibility of the moisture turning around at the end of the ventilation filter, reaching the rear end opening of the through hole through the gap between the upper surface of the grommet and the ventilation filter, and entering inside the sensor.
The invention was made in view of the above problems, and the object of the invention is to provide a technique for preventing moisture adhering to a ventilation filter from entering into the inside of a sensor along the surface of a ventilation filter.